This invention relates to an apparatus for reading color images by the use of a solid state image pickup device or the like, and more particularly to a color image reading apparatus for color-resolving and separating light from an object through an imaging optical system and color resolving means and directing the separated lights to sensors such as solid state image pickup devices.
An apparatus as shown in FIG. 1 of the accompanying drawings is known as an apparatus for line-scanning an object such as an original in the sub-scanning direction and color-reading the image thereof by a solid state image pickup device (such as CCD sensor) array. In FIG. 1, information light on a part of the surface 1 of an original irradiated with light from an illuminating light source (not shown) is resolved and separated into three colors by a three-piece (3P) prism 20 through an imaging optical system 19, whereafter the three color lights are imaged on three one-line CCD sensors 21, 22 and 23 and read.
However, in the above-described example of the prior art, three one-line sensors are independently necessary and usually, high accuracy of manufacture is required of the 3P prism 20, and this leads to high costs. Further, adjustment is independently necessary for the condensed light beam and each of the three sensors 21, 22 and 23, and this also has led to the disadvantage that the degree of difficulty of manufacture is high.
So, it would occur to mind to make three lines of sensor arrays on one and the same substrate in parallelism to one another with finite distances therebetween, and form three lines on an element as a monolithic three-line sensor.
Such a three-line sensor 24 is shown in FIG. 2 of the accompanying drawings. In FIG. 2, the distances S.sub.1 and S.sub.2 between three lines 25, 26 and 27 and are of the order of 0.1-0.2 mm from various manufacturing conditions, and the picture element widths W.sub.1 and W.sub.2 of each single element 28 are of the order of 7 .mu.m.times.7 .mu.m and 10 .mu.m.times.10 .mu.m.
A construction known as a color image reading apparatus using such a monolithic three-line sensor as a light receiving element is shown in FIG. 3 of the accompanying drawings.
In FIG. 3, when line-scanning the information on the surface 1 of the original in the sub-scanning direction and reading it, the light from the surface 1 of the original is resolved and separated into three light beams of three colors by color-resolving beam splitters 30 and 31 having selective transmission film of bicolor property added thereto, through an imaging optical system 29, whereafter the three light beams are condensed on respective corresponding line elements on a monolithic three-line sensor 32. As shown in FIG. 3, however, when the plate thickness of the beam splitters 30 and 31 is X, the inter-line distance on the sensor 32 is 2.sqroot.2.multidot.X, and when as previously described, the inter-line distance (2.sqroot.2.multidot.X) is set to the order of 0.1-0.2 mm, the plate thickness (X) is of the order of 35-70 .mu.m. This numerical value is not an easy value to obtain in manufacture when the degree of flatness of the surface required in performance is considered.
On the other hand, generally speaking, the distances of the other two lines 25 and 27 to the central lines 26 of the monolithic three-line sensor are usually equal in each of opposite directions and integer times as great as the picture element size (W.sub.2 in FIG. 2) in the sub-scanning direction.
This is for the following reason. As can be seen from FIG. 4 of the accompanying drawings, where an image is read by the above-described monolithic three-line sensor by the use of only an ordinary imaging optical system 45, the positions on the surface of the original, which are read at a time by the three lines 25, 26 and 27, are three different positions 25', 26' and 27' as shown in FIG. 4. Therefore, the signal components of three colors (R, G and B) relative to a certain position on the surface of the original are not read at the same time and thus, it is necessary to make them coincident with one another and combine them together after they are read.
For this purpose, the inter-line distances S.sub.1 and S.sub.2 are made integer times as great as the size W.sub.2 of each picture element and a redundant line memory conforming thereto is provided, and then G and R signals are delayed with respect, for example, to B signals, whereby a combined signal of three colors is obtained relatively easily. For this reason, the inter-line distances S.sub.1 and S.sub.2 are made integer times as great as the size W.sub.2, as described above. However, this means the provision of plural lines of expensive line memories to allot a redundant line memory to the inter-line distance, and this is very disadvantageous with respect to cost, and is far from providing an inexpensive color image reading apparatus.
Also, a color image reading apparatus using a brazed diffraction grating as color resolving means is known from U.S. Pat. No. 4,277,138 (corresponding DE 2645075).
However, in the construction disclosed in the above-mentioned publication, consideration is paid only to the light from a point on an object, and not to the so-called angle-of-field characteristic attributable to a finite reading width in the main scanning direction being present on the surface of the object.
Accordingly, it is an object of the present invention to provide an inexpensive and high-performance color image reading apparatus in view of the above-noted problems.
According to the present invention, in a line scan type color image reading apparatus having, for example, a monolithic three-line sensor in which one-dimensional sensor arrays, such as solid state image pickup devices, are disposed on one and the same substrate over three lines with finite distances therebetween in a direction perpendicular to the direction of the arrays and an imaging optical system for forming the image of an object on the sensors, there is achieved an inexpensive and high-performance line scan type color digital image reading apparatus. In the present invention a one-dimensional brazed diffraction grating for spectrally separating the light from the object by the diffraction effect and color-resolving the light into three colors in the direction perpendicular to the arrays and directing said color-resolved lights to the respective corresponding sensor arrays, is disposed in the optical path between said imaging optical path and said sensors. The number of steps of the diffraction grating having the staircase structure of said one-dimensional brazed diffraction grating is not limited to where three steps are necessary to efficiently divide the light into three colors, but is increased to four or more steps, whereby interference noise light, which is spectral sideband light, is prevented from mixing with each spectral color and good three-color spectral reading is made possible without respective wavelength selective filter characteristics (R, G and B) on the surfaces of the three-line sensors, which are usually set.